Ion source for mass filter wherein the electron and ion beam axes are the same



Nov. 30, 1965 KARL-GEORG GUNTHER 3,221,164

ION SOURCE FOR MASS FILTER WHEREIN THE ELECTRON AND ION BEAM AXES ARETHE SAME Filed Sept. 22, 1960 AMPLIFIE ORDER F{ g HIGH FREQUENCVGENERATOR Fig.3

MEASURING INSTRUMENT United States Patent() 3 221 164 IONSOURCE FOR Miss,FILTER WHEREIN THE g EcrRoN AND ION BEAM AXES ARE THE Karl-GeorgGiinther, Nurnberg, Germany, assignor to- Siemens-SchuckertwerkeAktiengesellschaft, Erlangen, Germany, a corporation of Germany FiledSept. 22, 1960, Ser. No. 57,851 Claims priority, application Germany,Sept. 25, 1959,

6 Claims. (Cl. 250--41.9)

plained in the above-mentioned copending application, is

so chosen that the ions passing through the periodic field travel eitheror stable or on instable paths depending upon their specific electriccharges. When the charges are such as to result in a stable path, theions will pass through the variable field of the deflector electrodesonto a collector electrode at the end of the beam-path axis. When theions travel on instable paths, they perform lateral pendulous motionsand are caught on the deflector electrodes.

Such mass-analyzing or isotope-separating devices are applicable aspartial-pressure vacuum meters, for detecting leaks in vacuum equipment,for the separation of gas mixtures, for trace analyses, or for measuringslight vapor pressures. The ion sources heretofore used in suchapparatus included an incandescent cathode as part of a discharge pathextending at .a right angle to the beam axis of the ions to be extractedfrom the source. Such transverse direction of the electron flow in thesource has heretofore been believed necessary for obtaining homogeneityof the energies of the generated ions.

It is an object of my invention to improve mass-filters of theabove-mentioned type toward greatly increasing the flow of ions from thesource toward the collector electrode, thus affording a simplificationof the indicating or measuring components to be used with the filteringapparatus proper, or improving the sensitivity of the mass filter sothat smaller amounts of mass quantities can be reliably observed.

To this end, and in accordance with my invention, I provide a massfilter of the above-described type with an electron-collision ion sourcehaving an electron discharge which is not self-sustaining, and whoseelectronbeam path has an axis coincident with the axis of the extractedion beam, the latter axis being determined by the axial spacing betweenthe ion source and the collector electrode and identical with the axisabout which the above-mentioned elongated deflector electrodes aredistributed.

Such an ion source is particularly advantageous at very low pressures ofthe gases to be investigated and results in a greatly increased quantityof ions passing from the source through the analyzing portion of theapparatus under otherwise comparable conditions. Such an ion source doesnot secure the above-mentioned degree of homogeneity with respect to theenergy contents of the extracted ions, but is based upon the recognitionand 3,221,164 Patented Nov. 30, 1965 observation that such homogeneityis not necessary in mass filters of the present type.

It is preferable to design the electrodes of the ion source in such amanner and to impress them with suitable voltages to obtain a repeatedpendulous travel of the ionizing electrons in the direction of the axisdefined by the extracted ion beam.

According to another feature of my invention, I provide the ion sourcewith focussing means which direct the extracted ions to the inletopening of the analyzing portion of the electric mass filter. Preferablyused for focussing is an electrostatic lens, although the same purposemay be served by a magnetic lens (as known, for example, from M. v.Ardenne, Tabellen derElektronenphysik, Ionenphysik und Uberrnikroskopie,vol. 2, page 803, Berlin 1956, published by Deutscher Verlag derWissenschaften) In order that the present invention may be readilycarried into effect, it will now be described with reference to theaccompanying drawings, in which:

FIG. 1 is a schematic block diagram of a complete mass-filter apparatuscorresponding to the one more fully illustrated and described in myabove-mentioned copending application Serial No. 859,030;

FIG. 2 is a side view, partly in section, of an embodiment of anelectron-collision ion source with which the apparatus is equipped inaccordance with the present invention and FIG. 3 is a view, partly insection, of an embodiment of a conventional electromagnetic lens whichmay be utilized with the apparatus of the present invention.

In FIG. 1, the mass filter is provided with an envelope structure 1which contains an ion source 2, a group of elongated deflectorelectrodes 3 consisting of rods and having individually a hyperboloidalor circular cross section. Located at the end of the ion-beam path is. acup-shaped collector electrode 4. The ion source 2 and the collectorelectrode 4 are coaxially spaced from each other and thus define acenter axis for the ion beam issuing from the source 2 toward theelectrode 4. The electrode rods 3 are uniformly distributed about theionbeam axis and extend parallel thereto. A total number of four suchelectrodes may be used.

The above-mentioned envelope 1 is vacuum-tight and has a neck portion 6connected with a tank or other vessel 7 which contains the gaseousmixture to be investigated. The rod electrodes 3 are electricallyconnected with a high-frequency generator 8 which supplies .theelectrodes with electric energy of suitable voltage and frequency. Thecurrent due to the ions impinging upon the collector 4 is amplified byan amplifier 9 and supplied to a recorder 10 or other indicating ormeasuring device. Another measuring instrument 11 is provided forsupervising the electron emission of the cathode in the ion source 2.

During operation, a beam of ions is continuously being extracted fromthe source 2 and is directed toward the collector 4. However, only theions of a given specific electric charge, or within a given range ofcharges, can reach the collector 4. Those ions which have differentspecific charges travel on instable, pendulous paths and thus impingeupon the deflector electrodes, thus being filtered out of the mixture.

The electron-collision ion source with which the apparatus is providedin accordance with the present invention is shown in FIG. 2 to include amounting 21 with to surround the incandescent cathode 22, and has acentral aperture in its bottom portion facing toward the anode 24. Theorientation of the electrode 23 is such that the electron beam, servingto effect ionization by collision, extends on the axis A of the ion beamto be extracted, this axis being identical with the above-mentioned axisdefined by the source 2 and the collector 4 (FIG. 1).

In order to focus the extracted ions onto the inlet opening of theanalyzing portion in the electric mass filter, the illustratedembodiment of the ion source is provided with an electrostatic lensalong the beam axis A between the anode 24 and the deflector-electrodeportion of the equipment. The electrostatic lens comprises a screenelectrode 25 having an aperture on the axis A, and another screen ordiaphragm member 26 which has a central aperture and forms the entrancediaphragm for the analyzing portion of the apparatus. The two components25 and 26, jointly constituting the electrostatic lens, are being keptat respectively different electric potentials. The voltages foroperating the source are indicated in FIG. 2 as: (ground); 0 to 50 v.;+50 to +200 v.; U ULI and UL2.

FIG. 3 illustrates an electromagnetic lens which may be utilized in theapparatus of FIG. 2. The electromagnetic lens comprises a winding 30positioned to coaxially surround the envelope 1 (FIG. 1) about the axisA of ion travel. A pair of apertures 33 and 34 are formed through coreportions 31 and 32, respectively, of the winding and are coaxial withthe axis A. The winding 30 is energized by means not shown in thedrawing.

With respect to other details and electric operating data beyond thedescribed features of the invention proper, reference may be had to theabove-identified earlier disclosures, such as the data of structuraldimensions, voltages and frequencies given in Patent 2,950,389. Incomparison with the mass filters according to the earlier disclosuresand under otherwise the same conditions, an apparatus according to theinvention, in which the electron beam serving to effect ionization bycollision extends in the axis of the extracted ion beam, yields agreatly increased ion current to pass into the ion separating portion ofthe apparatus, this increase being also due to the repeated pendulousmotion of the ionizing electrons within the source and by the bunchingefiect of the electric focussing lens.

While the invention has been described by means of a specific exampleand in a specific embodiment, I do not wish to be liimted thereto, forobvious modifications will occur to those skilled in the art withoutdeparting from the spirit and scope of the invention.

I claim:

1. In a mass filter having a negative-pressure envelope for receiving agas to be analyzed, an ion source and a collector electrode spaced fromeach other in said envelope and defining together an ion beam axis fromsaid source to said collector electrode, a group of elongatedion-defiector electrodes extending parallel to and uniformly spacedabout said axis between said source and said collector electrode, andhigh-frequency voltage supply means connected to said deflectorelectrodes for producing between them an electric field for ionseparation, the combination wherein said ion source comprises anelectron-emitting cathode, an anode spaced from said cathode along saidaxis toward said deflector electrodes and having a coaxial aperturetraversed when in operation by an electron beam for electron-collisionionization of said gas, said cathode and said anode aperture defining anelectron-beam axis coincident with said axis of the ion beam extractedfrom said source.

2. In a mass filter with an electron-collision ion source according toclaim 1, said cathode of said ion source comprising an incandescentmember, and a cup member coaxially surrounding said member, said cupmember having a bottom facing said incandescent member, said bottomhaving an aperture at its center coaxial with said incandescent member,said bottom being axially spaced from said member and from said anode.

3. A mass filter with an electron-collision ion source according toclaim 1, comprising a centrally apertured diaphragm mounted in saidenvelope transverse to said ion beam axis at a location between said ionsource and said deflector electrodes, and focussing means disposedbetween said source and said diaphragm for directing the extracted ionsthrough the aperture into the deflector-electrode space of the massfilter.

4. In a mass filter with an electron-collision ion source according toclaim 3, said focussing means consisting essentially of electro-opticallens means coaxially surrounding said ion-beam axis.

5. In a mass filter with an electron-collision ion source according toclaim 3, said focussing means comprising an electrostatic lens havingtwo centrally apertured lens members insulated from each other andextending transverse to said axis in axially spaced relation to eachother, one of said lens members being constituted by said diaphragm, andmeans for imposing respectively different electric potentials upon saidtwo lens members.

6. In a mass filter with an electron-collision ion source according toclaim 3, said focussing means consisting of an electromagnetic lens.

References Cited by the Examiner UNITED STATES PATENTS 2,468,261 4/1949Hillier 250--41.9 2,831,996 4/1958 Martina 250-419 2,939,952 6/1960 Paulet al. 250-419 RALPH G. NILSON, Primary Examiner.

ARTHUR GAUSS, Examiner.

1. IN A MASS FILTER HAVING A NEGATIVE-PRESSURE ENVELOPE FOR RECEIVING AGAS TO BE ANALYZED, AN ION SOURCE AND A COLLECTOR ELECTRODE SPACED FROMEACH OTHER IN SAID ENVELOPE AND DEFINING TOGETHER AN ION BEAM AXIS FROMSAID SOURCE TO SAID COLLECTOR ELECTRODE, A GROUP OF ELONGATEDION-DEFLECCTOR ELECTRODES EXTENDING PARALLEL TO AND UNIFORMLY SPACEDABOUT SAID AXIS BETWEEN SAID SOURCE AND SAID COLLECTOR ELECTRODE, ANDHIGH-FREQUENCY VOLTAGE SUPPLY MEANS CONNECTED TO SAID DEFLECTORELECTRODES FOR PRODUCING BETWEEN THEM AN ELECTRIC FIELD FOR IONSEPARATION, THE COMBINATION WHEREIN SAID ION SOURCE COMPRISES ANELECTRON-EMITTING CATHODE, AN ANODE SPACED FROM SAID CATHODE ALONG SAIDAXIS TOWARD SAID DEFLECTOR ELECTRODES AND HAVING A COAXIAL APERTURETRAVERSED WHEN IN OPERATION BY AN ELECTRON BEAM FOR ELECTRON-COLLISIONIONIZATION OF SAID GAS, SAID CATHODE AND SAID ANODE APERTURE DEFINING ANELECTRON-BEAM AXIS COINCIDENT WITH SAID AXIS OF THE ION BEAM EXTRACTEDFROM SAID SOURCE.